caddytls: Move initial storage clean op into goroutine
caddytls: Move initial storage clean op into goroutine

Sometimes this operation can take a while (we observed 7 minutes
recently, with a large, globally-distributed storage backend).

l3dlp/caddy,mholt/caddy,l3dlp/caddy,l3dlp/caddy,abiosoft/caddy,abiosoft/caddy,mholt/caddy